1. Field of the Invention
This invention relates generally to control of the operation of an electro-magnetic switch and, more particularly, to a method of and apparatus for controlling the supply of current to an exciting coil of an electro-magnetic switch in response to movements of a plunger.
2. Description of the Prior Art
A practical example of a conventional electro-magnetic switch is shown in FIG. 1. In FIG. 1, the supply of power to a load device 2 is controlled by closing and opening the electro-magnetic switch 3. The switch 3 is provided with a normally open contact circuit constituted by a pair of fixed contacts 3a, 3b and a movable contact 3c. The switch 3 also has an exciting coil 3d connected to a battery 1 through an operating switch 4 so that the exciting coil 3d can be energized to attract a plunger 5 in the direction indicated by an arrow in the drawing for movement of the movable contact 3c, thereby to cause the latter to abut against the fixed contacts 3a, 3b in opposition to the resilience of a return spring 3e and hold the switch 3 in its closed position.
The operation of the circuit shown in FIG. 1 is as follows. When the operating switch 4 is closed, the exciting coil 3d of the electro-magnetic switch 3 is excited to attract the plunger 5. Consequently, the movable contact 3c abuts against the pair of fixed contacts 3a, 3b in opposition to the action of the return spring 3e so that the power is supplied from the battery 1 through the electromagnetic switch 3 to the load device 2.
When the operating switch 4 is open, the exciting coil 3d is de-energized so that the movable contact 3c is returned under the action of the return spring 3e to the position shown in FIG. 1 to interrupt the connection between the battery 1 and the load device 2.
The conventional electro-magnetic switch is arranged in the manner mentioned above and, especially when it is used to interrupt a relatively large amount of current flow, the return spring has to provide a considerable spring force. Additionally, if the switch is provided in a place that is subjected to vibration, it will also be necessary for the return spring to be designed to generate sufficient spring force to prevent any malfunction of the contact. In any case, a large amount of current must be employed in operating the exciting coil and a large type of operating switch will also be needed. Moreover, the level of power consumption attributable to the exciting coil is high and this means that the switch device must be large in size due to thermal restrictions and the overall system is therefore large too which results in reduced electrical efficiency.
In FIG. 2, a curved line K.sub.1 or f.sub.1 -f.sub.2 shows variations in the attracting force of the plunger 5 with respect to the position of the plunger as it moves away from its rest position (point A) to its fully magnetically attracted position (point O). A line H.sub.1 or P.sub.1 -P.sub.2 also shows variations in the load of the return spring 3e, and a point B is the position of the plunger at the time when the contact closes. As is clear from FIG. 2, the force with which the plunger is attracted increases substantially along a quadratic curve from the rest position (point A), whereas the load of the return spring increases linearly in accordance with the spring constant.
In order for the plunger to be successfully attracted, it is necessary to establish the following condition at each of the positions A, B and O.